In many switch mode power converters, it is necessary or desirable to measure a current through an inductive element in the power converter.
In conventional switch mode power converters, measurement of this current is achieved by means of a sense resistor, located in series with the inductive element. The sense resistor is usually a separate component, and typically has a relatively low resistance of the order of 0.01 to 1 ohms, in order to minimise associated resistive losses. Nonetheless, and particularly for relatively low power outputs, these losses are undesirable.
One known solution to this problem is to position the sense resistor on the input side of the switch mode converter, in series with a power switch. Although positioning the resistance in series with the switch will cause lower losses, it may introduce other problems, particularly those associated with high-voltage operation. In order to determine the currents from the sense resistor, a current sense amplifier such as a trans-impedance amplifier is connected across the sense resistor: if the converter has a high input voltage, the current sense amplifier input needs to withstand that voltage, and also has two withstand any noise on the input supply line. A level shifter may be required in order to bring the output voltage of the amplifier down to ground level. Furthermore, any ringing on the switching node may introduce ringing at the current sensor, which may place a further high demand on the current sense amplifier.
A further, known, solution is to measure the voltage drop across the on-state drain-source resistance (Rdson) of the power switch, typically a high side switch in the case of a half bridge converter. However, such a solution is difficult to implement, and there is an ongoing need for an alternative solution.